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Think of this in this way: Murder 1 means someone planned this like, being resolved that they are going to do this before anything takes place. Murder 2 is being resolved to kill someone in the heat of the moment, like you weren't planning to kill someone today but in an event or just before you catch yourself resolved that you are going to kill this person. The difference is really the depth of planning, IIRC.

Zimmermann may not went into that car with a gun, thinking he's going to kill someone but he had pre-mediated that it is a possibility. And when he got Trayvon clearly feeling threatened, he took it upon him that Trayvon was doing something and escalated the situation that resulted killing him.